The invention is in the field of converting between pictorial information and electrical signals representing that information, and relates specifically to converting between images and Fourier transform representations thereof, and to utilizing such conversion.
Electronic processing of pictorial information is an active field, and there are many devices for converting between pictorial information and electrical representations thereof. The electrical representations are generally obtained by spatial scanning in which the electrical signal at any particular time represents the image intensity at a point on the image, e.g., TV-type scanning, but there are also devices which provide a Fourier transform representation of images. Such Fourier transform representation is desirable because it allows for more efficient and more versatile electronic processing of pictorial information, such as for improving image resolution, removing noise, providing electronic zoom, bandwidth compression, etc. In the most common approach to obtaining the Fourier transform of an image, a television camera scans an image spatially, point-by-point, and a computer computes a Fourier transform of the electrically represented light intensity pattern. Depending on the purpose of the information, the computer can compute the Fast Fourier, the Hadamand or other transforms. The difficulty with this approach is the amount of computation which must take place. For example, to compute the Fast Fourier transform of an image array that is 128 by 128 image points, it is necessary to perform over 200,000 computations, plus the reordering of a 128 by 128 complex-valued matrix. Assuming an average information retrievel and computation time of 4 microseconds per computation for a contemporary digital computer, this would take nearly a second. Other prior art approaches use vacuum TV camera tubes or charge coupled devices, but the difficulty of this is limited resolution. Still another approach is the use of optical lenses, but only the magnitude of the Fourier transform is obtained by this approach while the phase information is lost.
There is only one other prior art device known to applicants that may be able to generate electronically the Fourier transform of a one-dimensional pattern inversely proportional to the light intensity. This device has been developed recently at Stanford University by Professors Kino and Quate and consists of a piezoelectric substrate such as LiNiO.sub.3 with an interdigital transducer at each end and a silicon plate suspended 1000 Angstroms above the substrate. An electron current in the silicon plate interacts with the surface acoustic waves on the substrate. The acoustic waves are amplified wherever the electron drift velocity exceeds the surface wave velocity. Light incident on the silicon plate increases the available charge carriers and thus decreases the necessary charge carrier drift velocity to maintain a constant current. Thus, wherever the silicon plate is illuminated, the surface wave is amplified less. This process also allows the interaction of two acoustic waves. A signal corresponding to the amplitude of the Fourier transform of the light pattern on the silicon plate is sensed at one of the interdigital transducers. An essential part of this device is the 1000 Angstrom air gap between the silicon plate and the substrate, which makes it difficult to fabricate and to maintain in alignment.
There are difficulties with each of the prior art devices discussed above that obtain a Fourier transform (FT) representation of pictorial information, and there is still a need to obtain such representation in a more simple and a more efficient manner, so that FT represented images can find even wider use than now.
It is further desirable to find a way to convert from FT represented images to image intensity patterns directly rather than through the use of computers that calculate the inverse Fourier transform and drive a conventional CRT display.